I’m in the process of checking my V5 definitions in V6, and Sherlock Holmsing my way up to the origin of bugs.
Here’s one due to the stange way in which GH 1 outputs rectangle Base Planes from the “Deconstruct Rectangle” component :

The difference probably comes from the way the rectangles are made. Rectangles are defined as Planes and two numeric intervals (one along plane x-axis, one along plane y-axis). I’ll need the files that actually make the rectangles to see where it might be different from before.

I looked a bit upstream : my rectangles are created with the “Rectangular Grid” component.
Up to then, their bases are on the upper left.
Then they are scaled, and it seems that this is where they remain “normal” in V5 and become “funky” in V6.